Molten Salt Reactors vs India’s Advanced Heavy Water Reactor

One of the reactors India is working on is the AHWR, a solid thorium fueled, water cooled reactor. “AHWR is a 300 MWe, vertical, pressure tube type, boiling light water cooled, and heavy water moderated reactor. AHWR is being set up as a technology demonstration reactor keeping in mind the long term deployment of Thorium based reactors in the third phase. It will provide a platform for demonstration of technologies required for thorium utilisation. The reactor will use (Th-Pu) MOX and (Th-233U) MOX types of fuel. The fissile 233U for this reactor will be obtained by reprocessing its spent fuel, while plutonium will be provided from reprocessing of the spent fuel of PHWRs. The adoption of closed fuel cycle in AHWR helps in generating a large fraction of energy from thorium. A co-located fuel cycle facility (FCF) is planned along with the reactor and it will have facilities for fuel fabrication, fuel reprocessing and waste management. Some of the technologically challenging issues in this are handling of the highly radioactive fresh fuel, the requirement of remote fuel fabrication and carrying three-stream aqueous reprocessing by dissolution of the stable thoria matrix.” Also see the AHWR more info, “On an average, about 39% of the power is obtained from thorium.” The reactor has passive water cooling, with a pool of water above the reactor sufficient for 3 days “Later, cooling of the core is achieved by the injection of cold water from a large Gravity Driven Water Pool (GDWP) located near the top of the reactor building. In AHWR300-LEU, subsequent to energy absorption in GDWP in vapour suppression mode, the Passive Containment Cooling System (PCCS) provides long term containment cooling following a postulated LOCA. GDWP serves as a passive heat sink yielding a grace period of three days.”

The Molten Salt Reactor Experiment was designed and built in 5 years, with engineers using slide rules, with 1960s design tools, materials testing, etc. We have computer aided design, modern sensors, much better materials and materials testing facilities, computer nuclear modeling, flexible robotic assembly. You really think that with comparable number of people and budget to what Oak Ridge National Laboratories had, a better MSR would take more than the same 5 years?

I think in 5 years with proper staffing, we would have a reactor designed and factory designed and built to mass-produce MSR, with even better safety and fuel use than AHWR — MSR is a simpler design, if you include the fuel fabrication (none) and reprocessing (only fission products, no long term nuclear waste to deal with).

Fluorides are easier to chemically process than solid oxides; a required step in PUREX for LWR fuel is getting the fuel out of oxide). A two-fluid MSR such as LFTR wouldn’t have to separate thorium from the rare earth fission products, and simple fluorination returns uranium to the reactor. AHWR would have thorium, fission products, and uranium all in the same pellet.

AHWR is a solid fueled reactor, so there will be low fuel usage, though it will use less uranium and produce less actinides. Fission products trapped in fuel rods will stop fission long before the fuel is used. Most MSR designs would use over 99% of the fuel, and all fission products with long half-lives would remain in the reactor to decay by neutron bombardment; only isotopes with 35-year half lives or shorter as waste. MSR would have no need for “long-term storage of the spent fuel along with monitoring and retrieval” that AHWR will need.

AHWR is a water cooled reactor. Good that the passive safety features are so much better than LWR; but it still needs high-pressure systems for the cooling loop, and needs water after the 3 days the GDWP provides. Molten Salt Reactors are cooled by molten salts far below their boiling point, no high pressure systems needed at all. (Of course, the heat would be transferred outside the reactor to drive high pressure steam or other gas for the electric turbines.)

5 Comments

rahul ravindran
on May 10, 2015 at 3:26 am

has Govt. of India , commented on this particular issue ,regarding the use of Thorium in LFTR vs AHWR

It now 2017
From what I know – not much!
It just seems the molten Salt makes more sense – it needs recognition and tons of money for starters. Kirk Thorenson is trying with “Flibe” do you think his Co will get help to get going under this new Trump admin?

SO it can still go BOOM, like FKZ end CHRNBL, as it’s based on water for cooling… So needs massive containment and H2 management YES?

[No. Neither Chernobyl nor Fukushima had explosions because of water cooling. Water cooling requires safety equipment much more complex than salt cooling, and water cooling didn’t directly cause the damage in either site.

India’s AHWR would be a different type of nuclear reactor than Fukushima, and very different than Chernobyl. It would have much improved safety compared to the old LWR used at Fukushima. Chernobyl didn’t have any containment building at all.

Chernobyl didn’t explode because it had water coolant, the water pressure got so high because the fission rate got over 1000 times normal. Cheronbyl was designed with the potential of steam bubbles leading to rapidly increasing fission rate, illegal then except in USSR. Plus, the safety culture in USSR was not “safety above all”, but was “do what your manager orders you to, or you’ll be shot”. The Chernobly accident happened in 1986, but was an accident waiting to happen since it was built.

Fukushima had “massive containment” and had vents for releasing hydrogen if needed. The vents were designed to only be opened by motors, and the reactors and the backup diesel generators weren’t providing power for the motors (the reactors safely shut down, the diesel generators were ruined by tsunami). If the vents had been opened (how about a manual crank on a long pole to open the vents if needed?) there would have been a small release of radioactive materials into the air, not a hydrogen explosion ruining the containment building and spewing much more radiation into the air.

I don’t agree with building water-cooled nuclear power plants. Basically, only the fossil fuel companies want nuclear reactors using water coolant, so the fossil fuel industries won’t be replaced.

High Inherent Safety - No water, no high pressure, nothing that could propel radioactive materials into the environment. Thermal expansion/contraction of molten fuel salt strongly regulates fission rate; MSR is a very stable reactor. Simple safety systems work even if no electricity or operators.

Easy Construction and Siting - Low pressure operation, so no high-pressure safety systems. No water, so no steam containment building. Reactor factory assembled, with modern quality control, sensors and communication.